config: convert multi_replace to flags

config: convert multi_replace to flags

We will extend the flexibility of the config API. Before doing so, let's
take an existing 'int multi_replace' parameter and replace it with a new
'unsigned flags' parameter that can take multiple options as a bit field.

Update all callers that specified multi_replace to now specify the
CONFIG_FLAGS_MULTI_REPLACE flag. To add more clarity, extend the
documentation of git_config_set_multivar_in_file() including a clear
labeling of its arguments. Other config API methods in config.h require
only a change of the final parameter from 'int' to 'unsigned'.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>